Global-Scale Intraseasonal Oscillations of Outgoing Longwave Radiation and 250 mb Zonal Wind during Northern Hemisphere Summer

Abstract

Intraseasonal fluctuations of satellite-based observations of outgoing longwave radiation (OLR) and NMC analyses of 250 mb zonal wind (U250) are described based on global data from nine Northern Hemisphere summers (May?October). Cross-spectral analysis of the 28?72 day spectral band is used to establish statistically significant relationships for the entire data period. Hovmöller diagrams are used to examine individual events and to estimate the oscillation's time scale and propagation characteristics. Intraseasonal OLR fluctuations have their greatest amplitude in the Indian monsoon region and north of the equator in the western tropical Pacific. These two regions have out-of-phase fluctuations and appear to be linked by OLR anomalies propagating eastward (at 3?6 m s?1) along the equator between 50° and 160°E. The OLR oscillation has a preferred time scale of 30?60 days during May?October, based on a sample of more than 30 events. The initiation near the equator of northward-propagating (1?2 m s?1) OLR anomalies in the Indian monsoon region is also associated with the eastward-propagating equatorial OLR anomalies. The U250 intraseasonal fluctuations have a prominent zonal wavenumber-one structure throughout the tropics with the exception of the Northern Hemisphere tropics over the Atlantic, Africa, and the Indian monsoon region. The U250 anomalies propagate eastward along 0°?10°S at about 6 m s?1 from 40° to 160°E and at about 15 m s?1 from 160°E to 0°W. These longitudinal changes in the oscillation's ground speed may be due in part to longitudinal changes in the zonal wind basic state. The 28?72 day U250 anomalies along 30°S (50°S) are out of phase (in phase) with the tropical U250 anomalies over most of the Pacific and Indian Ocean sectors. The phase relationships between tropical OLR and U250 anomalies seem dynamically consistent, generally showing 250 mb u-component divergence flanking regions of convection. Although the eastward propagation of OLR anomalies along 5°N?5°S is not continuous around the globe, areas of significant coherence scattered throughout the tropics exhibit a zonal wavenumber-one phase structure. In these remote regions, OLR anomalies may be dynamically linked by an eastward-propagating tropical circulation feature.